As semiconductor geometries continue to shrink, manufacturers have increasingly turned to optical techniques to perform non-destructive inspection and analysis of semiconductor wafers. Techniques of this type, known generally as optical metrology, operate by illuminating a sample with an incident field (typically referred to as a probe beam) and then detecting and analyzing the reflected energy. Ellipsometry and reflectometry are two examples of commonly used optical techniques. For the specific case of ellipsometry, changes in the polarization state of the probe beam are analyzed. Reflectometry is similar, except that changes in intensity are analyzed. Ellipsometry and reflectometry are effective methods for measuring a wide range of attributes including information about thickness, crystallinity, composition and refractive index. The structural details of various metrology devices are more fully described in U.S. Pat. Nos. 5,910,842 and 5,798,837 both of which are incorporated in this document by reference.
As shown in FIG. 1, a typical ellipsometer or reflectometer includes an illumination source that creates a mono or polychromatic probe beam. The probe beam is focused by one or more lenses to create an illumination spot on the surface of the sample under test. A second lens (or lenses) images the illumination spot (or a portion of the illumination spot) to a detector. The detector captures (or otherwise processes) the received image. A processor analyzes the data collected by the detector.
Scatterometry is a specific type of optical metrology that is used when the structural geometry of a sample creates diffraction (optical scattering) of the incoming probe beam. Scatterometry systems analyze diffraction to deduce details of the structures that cause the diffraction to occur. Various optical techniques have been used to perform optical scatterometry. These include broadband spectroscopy (U.S. Pat. Nos. 5,607,800; 5,867,276 and 5,963,329), spectral ellipsometry (U.S. Pat. No. 5,739,909) single-wavelength optical scattering (U.S. Pat. No. 5,889,593), and spectral and single-wavelength beam profile reflectance and beam profile ellipsometry (U.S. Pat. No. 6,429,943). Scatterometry in these cases generally refers to optical responses in the form of diffraction orders produced by periodic structures, that is, gratings on the wafer. In addition, it may be possible to employ any of these measurement technologies, e.g., single-wavelength laser BPR or BPE, to obtain critical dimension (CD) measurements on non-periodic structures, such as isolated lines or isolated vias and mesas. The above cited patents and patent applications, along with PCT Application WO 03/009063, U.S. Application 2002/0158193, U.S. Application 2003/0147086, U.S. Application 2001/0051856 A1, PCT Application WO 01/55669 and PCT Application WO 01/97280 are all incorporated herein by reference.
To analyze diffraction, scatterometry systems use a modeling process. The modeling process is based on a parametric model of the particular sample being analyzed. The model is evaluated to predict the empirical measurements that a scatterometer will record for the sample. The predicted measurements and the empirical measurements are compared to determine if the model matches the empirical results. The model is then perturbed and re-evaluated until the predicted results and empirical results match within a desired goodness of fit. At that point, the parametric model is assumed to match the sample being analyzed.
As shown in FIG. 2, a typical scatterometry sample includes a scattering structure formed on a substrate. For the specific example of FIG. 2, the scattering structure is a grating composed of a series of individual lines. In general, the scattering structure may be periodic (as in the case of FIG. 2) or isolated. Isolated structures include, for example, individual lines or individual vias. The scattering structure of FIG. 2 is uniform (i.e., exhibits translational symmetry) along the Y axis. For this reason, this particular scattering structure is considered to be two-dimensional. Three dimensional scattering structures are also possible both in isolation (e.g., single via) or periodically (e.g., pattern of vias). The scatting structure is covered by an incident medium that is typically air but may be vacuum, gas, liquid, or solid (such as an overlaying layer or layers). One or more layers may be positioned between the scattering structure and the substrate. During analysis, a probe beam is directed at the scattering structure. For most applications, the probe beam intersects the scattering structure at a normal angle—it is perpendicular to the lines from which the scattering structure is formed. It is also possible to use a non-normal angle. This is referred to as conical scattering.
In practice, it is not generally possible to construct semiconductor wafers with the degree of orthogonality shown in FIG. 2. This is due to a number of physical limitations, such as the accuracy of the equipment used during fabrication. The overall result is that the scattering structures typically included in semiconductor wafers tend to have sloping instead of vertical walls, rounded corners at the foot and top of lines and a range of other artifacts introduced during the fabrication process. As semiconductor features continue to shrink, molecular size introduces a second type of non-orthogonality into the fabrication process. This second type of non-orthogonality arises because the molecules used to form the features of semiconductors become increasingly large (in a relative sense) as the features become increasingly small. As a result, small features tend to exhibit a non-uniformity, or roughness, caused by the physical size of their constituent molecules. This is particularly true where organic photo-resists are used. FIG. 3 shows a specific example where the use of relatively large molecules has resulted in line edge roughness.
Non-uniformity, or roughness of the type shown in FIG. 3, changes the measurements recorded during the scatterometry process. This is problematic because the models used to predict the empirical scatterometry measurements are not designed in a way that predicts the type of measurements that are associated with rough edges or other non-uniformities. For many modeling techniques, this problem is exacerbated because they assume that the scattering sample is two-dimensional (as is the case for the sample of FIG. 2).
For these reasons and others, a need exists for scatterometry techniques that are compatible with samples having rough or non-uniform edges. This need is particularly apparent for high density semiconductor wafers where feature sizes are small and particularly apparent where organic photo-resists are used.